The interrupting power of an SF.sub.6 circuit-breaker depends to a great extent on the pressure inside the circuit-breaking chamber since pressure has a direct effect on the mass flowrate of the SF.sub.6 gas used for extinguishing the arc. As temperature falls, so does the pressure of the gas and the mass flowrate is greatly reduced, which can lead to arcs re-striking while the circuit-breaker is interrupting an electric current.
U.S. Pat. No. 4,273,978 describes an SF.sub.6 circuit-breaker in which a piston urges liquid SF.sub.6 into the contact zone when the circuit-breaker is opened. This arrangement increases the mass flowrate of SF.sub.6 at the moment of extinction, but it requires very high operating energy. An aim of the invention is to obtain a high mass flowrate of SF.sub.6 when interrupting a current, without requiring the operating energy for opening the circuit-breaker to be increased.